Question #bb8ec

Apr 13, 2017

You can make phosphate buffers for pH 2, 7, and 12.

Explanation:

Phosphoric acid has three dissociation constants.

(1) $\text{H"_3"PO"_4 + "H"_2"O" ⇌ "H"_3"O"^+ + "H"_2"PO"_4^"-"; "p} {K}_{\textrm{a 1}} = 2.15$

(2) $\text{H"_2"PO"_4^"-" + "H"_2"O" ⇌ "H"_3"O"^"+" + "HPO"_4^"2-"; "p} {K}_{\textrm{a 2}} = 7.20$

(3) $\text{H"_3"PO"_4^"2-" + "H"_2"O" ⇌ "H"_3"O"^"+" + "PO"_4^"3-"; "p} {K}_{\textrm{a 1}} = 12.35$

Thus, you could make a buffer of pH 2 by preparing a solution of phosphoric acid and sodium dihydrogen phosphate (Equation 1).

You could make a buffer of pH 7 by preparing a solution of sodium dihydrogen phosphate and sodium hydrogen phosphate (Equation 2).

You could make a buffer of pH 12 by preparing a solution of sodium hydrogen phosphate and sodium phosphate (Equation 3).

The ranges for each buffer would be $\text{p"K_text(a) ± "1 pH unit}$.